The invention concerns a method and a device for optical measurement of an electrical voltage, preferably a high voltage.
Conventional voltage transformers used for measuring high voltages in power technology installations are based on an inductive measurement principle; capacitive voltage dividers may also be used in addition. In conventional transformers, expenditure on insulation increases at a disproportionately high rate in relation to the transmission voltage of the power supply network (see A. J. Schwab, “Hochspannungsmeβtechnik [High-Voltage Measuring Techniques]”. Electromagnetic compatibility (EMC) has gained in importance in the course of increasing digitalization of the measuring technology following the transformers, wherein this measuring technology generally has lower interference thresholds than conventional analog measurement technology. Because of the inductive-capacitive coupling of the primary plane (supply side) to the secondary plane (measurement and control side) in conventional voltage transformers, their use in connection with digital network technology turns out to be problematic as concerns EMC (see H. Hirsch, “Polarimetrische faseroptische Stromwandler [Polarimetric fiber-optic current transformers]”). Compared to conventional transformers, little raw material is used owing to the small size of optical component assemblies. Optical transformers do not require any oil for insulation in principle, so that the risk of contamination of adjoining soil with oil in the event of a transformer explosion due to defect on the network side or device side is nonexistent.
Optical measuring methods in which electrical fields and electrical voltages are measured via the Pockels effect in electro-optical crystals are already generally known from different references. In this connection, the physical properties of an electro-optical medium change as a function of the electrical field strength in such a way that the polarization state of the optical wave propagating through the sensor medium is influenced by a linear birefringence induced by the electrical field. With the help of an optical arrangement comprising a polarizer, a delay element, an electro-optical material and an analyzer, in combination with electronic evaluating means, the measurement signal can be acquired for determining the electrical voltage transverse or parallel to the propagation direction of the optical wave. In order to make it possible to separate the useful quantity—electrical voltage—from the interference quantities—damping along the optical signal path which is not constant over time, temperature dependencies on parameters of the optical components employed—the optical signal path is divided into more than one partial beam. The partial beams are guided to separate receivers via different optical elements and the detected signals are, if necessary, subjected to digital signal processing after suitable processing through analog electronic means.
In DE 4436454, polarized measurement light is guided through a Pockels sensor device under the influence of the alternating field or AC voltage to a beam splitter which splits the optical wave into two different polarization planes. The method indicated in the embodiment form makes use of the transverse electro-optical effect (FIG. 1) for measuring the electrical field. The method is suitable for measurement of voltages which drop transversely across the sensor crystal. It is possible to adapt the measurement range by changing the crystal length, but the maximum measurable voltage is limited by the electrical strength of the sensor crystal. Due to the fact that the crystal dimensions are limited in practice, the measurement of high voltages by means of the transverse electro-optical effect is very uneconomical in technical respects; however, the measurement of “small” voltages below the electrical strength of the crystal material through an increase in sensitivity by lengthening the crystal is useful.
DE-OS 4416298 describes an embodiment form of the measurement process and the device for carrying out the process which make use of the longitudinal electro-optical effect. An electric voltage to be measured generates an electrical field in the crystal whose flux lines run parallel to the propagation direction of the measurement light. Due to the maximum technically possible crystal dimensions and the limited electrical strength in this connection, there is a considerable increase in expenditure on insulation for measurements of electrical voltages in the range of the maximum electrical strength of the arrangements.
German Patent 4100054 proposes an optical measurement transducer which supplies a measurement for electrical current by determining the magnetic field and, by means of an installed capacitive divider, makes use of the voltage drop across a partial capacitance as a measurement for electrical voltage. The electrical voltage is exactly determined only when the indicated splitting ratio determined by overvoltage capacitance and undervoltage capacitance remains constant. Since a spatially expanded undervoltage capacitance is used, the capacitance can be influenced by field distortion, so that the splitting ratio of the measurement transducer is changed. In general, constant field distributions cannot be assumed in practice.
DE-EB 3404608 describes a device for optical measurement of the electrical field strength which supplies, via a transmission element, an optical wave of a sensor device for an electrical field which changes the degree of modulation of the optical wave depending on the electrical field strength. It is noted that the utilized sensor crystals from groups 23 and 43m exhibit a limited dependency of the optical effect on temperature, but there is no complete compensation of the temperature influence.
A device for measuring a voltage and an electrical field using light is indicated in DE Patent 3039136. The patent describes the use of, e.g., a bismuth-germanium oxide crystal for voltage measurement and field measurement. It is indicated that the temperature dependencies of the material-specific constant Vn can be assumed at about 0.01% /K. Consequently, in a temperature range of ΔT=100K the error can amount to 1%. For applications with higher accuracies, it is necessary to compensate for the temperature characteristic not only of the sensor crystal, but also of the delay plate.
DE-EB 2845625 describes an arrangement for electro-optical voltage measurement which makes use of the longitudinal linear electro-optical effect of a piezoelectric fiber and in which there is effected an integration of the optical effects of the field strength distribution along the fibers by means of the spatial dimensioning of the crystal fibers. According to the state of the art, a crystal fiber of this type is not currently commercially available, so that this method of voltage measurement has not been successful in practice in technical respects relating to large-series manufacture.
DE-EB 2131224 discloses a device for measurement of voltages of high-voltage conductors in which it is indicated that the electrical field proportional to the voltage to be measured changes the polarization plane of polarized light which is coupled into a light waveguide. In a suggested arrangement, the light waveguide is guided along a curvy path in order to increase the effect. A high temperature dependency of the measurement signal caused by the linear birefringence of the light waveguide induced by bending would be expected in this embodiment.
DE-EB 1591976 describes an electro-optical voltage reducing device and its application for measuring voltages. In this case, the polarization of a light bundle traversing a quantity of electro-optical cells which are electrically connected in series is changed and read out by a Pockels cell via a compensating circuit. In principle, the described arrangement is a resistive-capacitive splitter whose voltage drops across partial capacitances are read out optically. The method has the disadvantage that temperature dependencies of the optical elements are not compensated and that the suggested device is uneconomical in technical respects and is accordingly expensive to produce because the cost of the voltage divider is added to that of the optical construction. Further, the compensation circuit necessitates supply of a secondary electrical voltage.
DE 4436181 A1 discloses a method and a device for measuring an electrical AC quantity with temperature compensation through fitting. A suggesting scaling circuit takes the ratio of AC signal component to DC signal component of the intensity signal of the optical wave detected by the receivers. A divider is used to carry out this function. No steps are indicated for suppressing the effects of tolerances of the structural component parts in the scaling stage.